Merge remote-tracking branch 'origin/master' into release-1.28

Author: k8s-release-robot

build/root/Makefile

@@ -257,6 +257,8 @@ define TEST_E2E_NODE_HELP_INFO
 #  SSH_USER: For REMOTE=true only  SSH username to use.
 #  SSH_KEY: For REMOTE=true only. Path to SSH key to use.
 #  SSH_OPTIONS: For REMOTE=true only. SSH options to use.
+#  INSTANCE_TYPE: For REMOTE=true only. Machine type to use.
+#  NODE_ENV: For REMOTE=true only. Additional metadata keys to add the instance.
 #  RUNTIME_CONFIG: The runtime configuration for the API server on the node e2e tests.
 #
 # Example:

test/e2e_node/remote/gce/gce_runner.go

@@ -58,6 +58,9 @@ func (e *envs) String() string {
 
 // Set function of flag.Value
 func (e *envs) Set(value string) error {
+	if value == "" {
+		return nil
+	}
 	kv := strings.SplitN(value, "=", 2)
 	if len(kv) != 2 {
 		return fmt.Errorf("invalid env string %s", value)